The present invention relates to a method and device for coding digital signals, to a method and device for decoding these same digital signals, and to systems using them.
The digital signals in question can be images, video signals, sound signals or data. Here the invention will be described particularly in its application to digital signals representing images.
In the field of digital signal coding, it is sometimes advantageous to use several coding techniques for coding the same set of data. In such case, by virtue of a selection method, based for example on an optimum transmission rate allocation criterion, the best coding technique is chosen and applied locally.
For the decoder to be able to correctly decode the coded signals which it receives, an item of information referred to as a xe2x80x9cflagxe2x80x9d is, conventionally, generally transmitted, making it possible to know which coding technique has been used on each signal portion.
Thus, for example, in accordance with the prior art, a digital image can be divided into blocks, and, for coding each block, it is possible to have a choice between coding by vector quantisation and coding by discrete cosine transform (DCT). Each block is coded by the most appropriate method within the terms of a criterion fixed a priori, and a flag is transmitted with the image signal, in order to indicate, for each block, whether vector quantisation or DCT has been used.
The flag is for example a word able to take as many different values as there are different coding techniques used for coding the blocks.
This way of proceeding according to the prior art has notably the drawback of requiring the transmission, from the coder to the decoder, of a large amount of information in addition to the coded signal proper, and therefore of reducing the compression rate of the signals, which consequently limits the transmission rate.
In an article entitled xe2x80x9cHigh compression image coding using an adaptive morphological sub-band decompositionxe2x80x9d, in Proceedings of the IEEE, vol. 83, No 2, February 1995, O. Egger, W. Li and M. Kunt proposed a technique for coding an image signal using an adaptive decomposition of the signal, which uses either linear filters, for the textured regions of the image, or morphological filters, for the other regions of the image. No xe2x80x9cflagxe2x80x9d is transmitted to the decoder.
On the other hand, this method of the prior art has the drawback of requiring the coder and decoder to both calculate the local variance for each pixel of the original image, in order to determine whether or not the region in question is a textured region, knowing that a texture has a high local variance in all directions.
This method therefore has the drawback of entailing high calculation cost due to the seeking of textured regions, given that the decision on the textured character is made pixel by pixel.
The aim of the present invention is to remedy the aforementioned drawbacks of the prior art, by eliminating the need to transmit, from the coder to the decoder, additional explicit information indicating the coding technique used and lessening the calculation cost, whilst making it possible to improve the compromise between the compression and distortion rates of the signals.
To this end, the present invention proposes a method of coding a digital signal, characterised in that:
(a) a signal simplification step is performed, during which a simplified version of the digital signal is determined, containing only part of the information contained in this digital signal;
(b) a step of coding the simplified signal is performed, during which the simplified version is coded by means of a predetermined single coding technique;
(c) a subtraction step is performed, during which this simplified version is subtracted from the digital signal so as to obtain a residual signal;
(d) a coding selection step is performed, during which, on the basis of each block of the simplified version, one coding technique, amongst a predetermined number M of coding techniques Ci, where i is an integer between 1 and M, is selected for each corresponding block of the residual signal;
(e) a residual signal coding step is performed, during which each block of the residual signal is coded by means of the previously selected coding technique.
Thus the choice of the coding technique is made on the basis of the simplified version of the original signal, and no flag directly bearing a mention of the coding technique used is transmitted from the coder to the decoder.
Given that the simplified version of the original signal contains a smaller quantity of information compared with the original signal, it offers great simplicity of coding. In addition, this simplified version being a subset of the total information contained in the original signal, the addition cost, compared with the cost necessary for coding the image in its totality, is nil, or low where there exists redundancy between the simplified version and the residual signal. This thus makes it possible to save on all or part of the transmission rate which, according to the prior art, was reserved for the transmission of flags.
The above simplified signal coding step (b) can be performed either at the end of the steps (a) of simplification, (c) of subtraction, (d) of coding selection or (e) of residual coding. This gives the present coding method great flexibility of implementation.
In a particular embodiment where the digital signal is an image signal, according to a first variant, the aforementioned simplified version can have the same spatial resolution as the original digital signal. For example, it can be a simplified image obtained by opening-closing followed by morphological reconstruction. For more details on the concept of morphological reconstruction, reference can usefully be made to the article by P. Salembier entitled xe2x80x9cMorphological multiscale segmentation for image codingxe2x80x9d, in Signal Processing, 1994, No 38, pages 359-386.
This type of operators had the advantage of eliminating all the objects smaller than a certain size, and of restoring the contours of the objects which have not been completely eliminated. This still further facilitates the coding of the simplified image.
In this particular embodiment, according to a second variant, the simplified version can have a spatial resolution less than that of the original signal.
Such a simplified image can for example have been obtained by wavelet decomposition of the original signal and definition of the simplified image as being a sub-band obtained by low-pass filtering during this wavelet decomposition.
The obtaining of such a simplified image has the advantage of guaranteeing a concentration, in the aforementioned low sub-band, of the information contained in the original image.
According to a first aspect of the coding method of the present invention, at the coding selection step (d), a prediction step is performed during which, on the basis of the simplified version, the most appropriate coding technique amongst the M coding techniques Ci is predicted for each block of the digital signal, and in addition steps are performed according to which:
(f) an optimised coding selection step is performed, during which, using a predetermined optimisation criterion, the most appropriate coding technique amongst the M coding techniques Ci is selected for each block of the residual signal, and this block is coded by means of the coding technique selected;
(g) a comparison step is performed, during which, for each block of the digital signal, the coding technique predicted at the coding selection step (d) and the coding technique selected at the optimised coding selection step (f) are compared; and
(h) a storage step is performed, during which, if the predicted and selected coding techniques are identical, there is stored, with a view to decoding, a flag according to which the prediction made at the coding selection step (d) is correct, and otherwise a flag is stored according to which this prediction is false.
Thus there is transmitted, from the coder to decoder, not directly information relating to the coding technique used for each block of the signal, but more condensed information, able to be coded by means of a single bit, giving information on the validity of the prediction of the coding technique. This makes it possible to arrive at a compromise between the compression and distortion rates of the signals more advantageous than those proposed by the prior art.
According to the aforementioned first aspect, the optimisation criterion can consist of maximising the ratio of transmission rate to distortion, which effectively helps to obtain an advantageous compromise between the compression and distortion rates of the signals.
According to a second aspect of the coding method of the invention, according to which the digital signal is an image signal and the M coding techniques Ci comprise a technique of coding with losses of information and a technique of coding without losses of information, at the coding selection step (d), the presence of contours is sought in each block of the simplified version, and then
(d1) a coding step without losses is performed, during which, if the block under consideration contains at least one contour, the corresponding block of the residual signal is coded by means of the coding technique without losses of information; or
(d2) a step of coding with losses is performed, during which, if the block under consideration contains no contour, the corresponding block of the residual signal is coded by means of the coding technique with losses of information.
The contour detection used here is particularly advantageous through its simplicity and through the high-performance nature of several contour detection techniques well known to persons skilled in the art. Moreover, if it is chosen to seek the contours, this is because errors on contours are more visible than errors on textures.
For the same purpose as mentioned above, the present invention also proposes a method of decoding a coded digital signal, this coded signal coming from an original digital signal decomposed into a simplified version and a residual signal, obtained by subtracting the simplified version from the original digital signal, the simplified version being coded by means of a predetermined single coding technique, and the residual signal being coded by means of a coding technique selected from amongst a predetermined number M of coding techniques Ci, where i is an integer between 1 and M, characterised in that:
a simplified signal decoding step is performed, during which the simplified version is decoded by means of the decoding technique associated with the predetermined single coding technique; and
a decoding selection step is performed, during which, on the basis of each block of the decoded simplified version, there is selected, for each corresponding block of the residual signal, a decoding technique taken from amongst a predetermined number M of decoding techniques Di respectively associated with the M coding techniques Ci;
a residual signal decoding step is performed, during which each block of the residual signal is decoded by means of the decoding technique which was selected for this block; and
an addition step is performed, during which the decoded simplified version is added to the decoded residual signal, so as to reconstitute the original digital signal.
According to the first aspect of the invention, during the decoding selection step,
a prediction step is performed, during which, on the basis of each block of the decoded simplified version, there is predicted, for the corresponding block of the residual signal, the most appropriate decoding technique associated with one of said M coding techniques Ci;
a reading step is performed, during which a flag associated with the block of the simplified version under consideration is read, relating to the validity of the prediction of the coding technique for the corresponding block of the digital signal;
a residual signal decoding step is performed, during which, if and only if, according to this flag, the prediction is correct, the corresponding block of the residual signal is decoded by means of the predicted decoding technique.
According to the second aspect of the invention, during the decoding selection step, the presence of contours is sought in each block of the decoded simplified version, and then
a decoding step without losses is performed, during which, if the block under consideration contains at least one contour, the corresponding block of the residual signal is decoded by means of the technique of decoding without losses of information associated with the technique of coding without losses of information; or
a decoding step with losses is performed, during which, if the block under consideration contains no contour, the corresponding block of the residual signal is decoded by means of the technique of decoding with losses of information associated with the technique of coding with losses of information.
The present invention also proposes a device for coding a digital signal, characterised in that it has:
simplification means, for determining a simplified version of the digital signal, containing only some of the information contained in the digital signal;
first coding means, for coding the simplified version by means of a predetermined single coding technique;
subtraction means, for subtracting the simplified version from the digital signal, so as to obtain a residual signal;
coding selection means, for selecting, on the basis of each block of the simplified version, for each corresponding block of the residual signal, a coding technique amongst a predetermined number M of coding techniques Ci, where i is an integer between 1 and M; and
second coding means, for coding each block of the residual signal by means of the coding technique selected by the selection means.
According to a particular characteristic, the coding selection means include prediction means for predicting, on the basis of the simplified version, for each block of the digital signal, the most appropriate coding technique amongst the M coding techniques Ci.
The present invention also proposes a device for decoding a coded digital signal, the coded signal coming from an original digital signal decomposed into a simplified version and a residual signal, obtained by subtracting the simplified version from the original digital signal, the simplified version being coded by means of a predetermined single coding technique, and the residual signal being coded by means of a coding technique selected from amongst a predetermined number M of coding techniques Ci, where i is an integer between 1 and M, characterised in that it has:
first decoding means, for decoding the simplified version by means of the decoding technique associated with the predetermined single coding technique;
decoding selection means, for selecting, on the basis of each block of the decoded simplified version, for each corresponding block of the residual signal, a decoding technique taken from amongst a predetermined number M of decoding techniques Di respectively associated with the coding techniques Ci;
second decoding means, for decoding each block of the residual signal by means of the decoding technique selected for this block by the selection means; and
adding means, for adding the decoded simplified version to the decoded residual signal, so as to reconstitute the original digital signal.
According to a particular characteristic, the decoding selection means have prediction means for predicting, on the basis of each block of the decoded simplified version, for the corresponding block of the residual signal, the most appropriate decoding technique associated with one of the M coding techniques Ci.
The invention also relates to:
any digital signal processing apparatus having means adapted to implement a coding method as succinctly disclosed above;
any digital signal processing apparatus having a coding device as succinctly disclosed above;
any digital signal processing apparatus having means adapted to implement a decoding method as succinctly disclosed above; and
any digital signal processing apparatus having a decoding device as succinctly disclosed above.
The invention also relates to:
an information storage means which can be read by a computer or a microprocessor storing instructions of a computer program, making it possible to implement the coding and/or decoding method of the invention as succinctly disclosed above, and
an information storage means which is removable, partially or totally, and which can be read by a computer or a microprocessor storing instructions of a computer program, making it possible to implement the coding and/or decoding method as succinctly disclosed above.
The particular characteristics and the advantages of the coding device, of the decoding method and device, of the digital signal processing apparatus and the information storage means being the same as those of the coding method of the invention, they are not repeated here.
The invention also proposes a method of coding a digital signal, comprising the steps of:
generating a simplified signal and a residual signal from the digital signal;
determining a coding technique for said residual signal among a set of coding techniques on the basis of a feature of said simplified signal;
coding said residual signal with said determined coding technique and said simplified signal with a predetermined coding technique.
According to a particular characteristic, the method further comprises the step of:
dividing said simplified signal and said residual signal into a plurality of blocks respectively.
According to a particular characteristic, said determining step determines coding techniques on the basis of each block of said simplified signal for each corresponding block of said residual signal.
According to a particular characteristic, said simplified signal is generated by a wavelet transformation of the digital signal.
The invention also proposes a method of decoding a signal coded by means of a coding method as succinctly disclosed above, comprising the steps of:
decoding the coded simplified signal;
determining a decoding technique for said coded residual signal among a set of decoding techniques on the basis of a feature of said decoded simplified signal;
decoding said coded residual signal with said determined decoding technique.
According to a particular characteristic, said determining step determines decoding techniques on the basis of each block of said decoded simplified signal for each corresponding block of said coded residual signal.
The invention also proposes a device for coding a digital signal, adapted to implement a coding method as succinctly disclosed above.
The invention also proposes a device for decoding a digital signal, adapted to implement a decoding method as succinctly disclosed above.
The invention also relates to a digital signal processing apparatus, adapted to implement a coding method as succinctly disclosed above.
The invention also relates to a digital signal processing apparatus, adapted to implement a decoding method as succinctly disclosed above.